Is anyone else see file copies failing under load?
If so please take a look & comment/vote on this bug -> https://connect.microsoft.com/feedback/ViewFeedback.aspx?FeedbackID=289490&SiteID=168
Right now I' m (unfortunately) finding I have to enable realtime checking to do anything involving lots of copies -- clearly dangerous to force me to de-install and go back to Avast.
No, it is not you. That is actually a Vista problem and not limited to OneCare. There have been many reports of this for a variety of antivirus programs.
It is still worthwhile filing the bug against OneCare, so thanks for doing so.